1. Field of the Invention
The present invention relates to a high strength titanium alloy which has high strength, excellent weldability (i.e., ductility in heat affected zone (HAZ) after welding, the same meaning hereinafter) and good ductility to make the production of strips possible. The present invention relates to a titanium alloy coil-rolling process and a process for producing a coil-rolled titanium strip, in which the titanium is the above-mentioned titanium alloy.
2. Related Art
Titanium and its alloys are light, and excellent in strength toughness and corrosion-resistance. Recently, therefore, they have widely been made practicable in the fields of the aerospace industry, the chemical industry and the like. However, titanium alloys are materials which are generally not so good in workability, so that costs for forming and working are very high, as compared with other materials. For example, Tixe2x80x946Alxe2x80x944V, a typical xcex1+xcex2 type alloy, is a material which is difficult to work at room temperature. Thus, it is said that the alloy can hardly be made into a coil by cold rolling.
For this reason, at the time of rolling the Tixe2x80x946Alxe2x80x944V alloy into a sheet form, a manner called pack-rolling is adopted. That is, the pack-rolling is a manner of stacking Tixe2x80x946Alxe2x80x944V alloy sheets obtained by hot rolling in the form of layers, putting the sheets into a box made of mild steel, and hot rolling the sheets packed into the box under heat-retention for keeping its temperature more than a given temperature to produce a thin plate. In this process, however, a mild steel cover for making a pack and pack welding are necessary. Moreover, in order to block bonding of titanium alloy strips themselves, a releasing agent must be applied. In such a manner, the pack-rolling process requires very troublesome works and great cost, as compared with cold rolling. Additionally, the temperature range suitable for hot rolling is limited, to cause many restrictions in working.
On the contrary, Japanese Patent Application Laid-Open Nos. 3-274238 and 3-166350 discloses that the contents of Al, V and Mo in the parent material of titanium are defined and at least one alloying element selected from Fe, Ni, Co and Cr is comprised therein in an appropriate amount, so that a titanium alloy can be obtained which has a strength substantially equal to that of the Tixe2x80x946Alxe2x80x944V alloy and are superior to the Tixe2x80x946Alxe2x80x944V alloy in superplasticity and hot workability.
Japanese Patent Application Laid-Open Nos. 7-54081 and 7-64083 disclose a titanium alloy in which the Al content is reduced up to a level of 1.0-4.5%, the V content is limited to 1.5-4.5%, the Mo content is limited to 0.1-2.5%, and optionally a small amount of Fe or Ni is comprised thereinto, thereby keeping high strength and raising cold workability and weldability (in particular, HAZ after welding).
This titanium alloy has both cold workability and high strength, and further has improved weldability, and thus is an excellent alloy. However, in these inventions, flow-stress during plastic deformation is suppressed because of the necessity of ensuring excellent cold workability. Thus, its strength is considerably low. If the strength is raised, its cold workability drops. For this reason, production of cold strips are substantially impossible. Incidentally, in recent years, customers"" demands of high strength and high ductility to titanium alloys have been becoming more and more strict. Thus, titanium alloys are desired to be improved still more.
Paying attention to the above-mentioned situation, the inventors have made the present invention. The subject of the present invention is an xcex1+xcex2 type titanium alloy, and an object thereof is to provide an xcex1+xcex2 type titanium alloy having excellent strength and cold workability, and further having ductility making it possible to produce strips in coil. Another object of the present invention is to establish a continuous rolling technique based on coil-rolling by devising working conditions, and provide a process for obtaining a titanium alloy having excellent workability and strength by annealing after the coil-rolling.
The high strength and ductility xcex1+xcex2 type titanium alloy of the present invention for overcoming the above-mentioned problems comprises at least one isomorphous xcex2 stabilizing element in a Mo equivalence of 2.0-4.5 mass %, at least one eutectic xcex2 stabilizing element in an Fe equivalence of 0.3-2.0 mass %, and Si in an amount of 0.1-1.5 mass %. (Hereinafter, % means % mass unless specified otherwise.) In the titanium alloy, a preferred Al equivalence, including Al as an xcex1 stabilizing element, is more than 3% and less than 6.5%. If C is further comprised thereinto in an amount of 0.01-0.15%, the strength property of the alloy becomes more excellent.
The process for coil-rolling relates to a coil-rolling process which is suitable for the above-mentioned titanium alloy and makes continuous production possible. The process comprises annealing a strip of the titanium alloy at a temperature satisfying the following inequality [1], and then coil-rolling the resultant.
(xcex2 transusxe2x88x92270xc2x0 C.)xe2x89xa6Txe2x89xa6(xcex2 transusxe2x88x9250xc2x0 C.) xe2x80x83xe2x80x83(1) 
At the time of the coil-rolling, preferably the tension for the coil-rolling ranges from 49 to 392 MPa and the rolling ratio for the coil-rolling is 20% or more. If the coil-rolling is performed plural times in a manner that an annealing step in the xcex1+xcex2 temperature range intervenes therebetween, the total rolling reduction can be raised as the occasion demands. Thus, even a thin plate can easily be obtained.
Furthermore, the process for producing a titanium alloy strip according to the present invention is a process of specifying annealing suitable for cold-rolled strips after the cold-rolling of the above-mentioned xcex1+xcex2 type titanium alloy. The process is characterized by improving transverse elongation of a cold-rolled titanium strip by selecting a heating temperature at the time of annealing from temperatures which are not less than temperature for relieving work-hardening at the time of cold-rolling and are temperatures, in the range of temperatures not more than xcex2 transus (Txcex2), for promptly avoiding temperature ranges causing brittleness resulting from the formation of brittle hexagonal crystal xcex1, so as to perform the annealing.
The above-mentioned titanium alloy is used to perform the annealing, so as to easily obtain a titanium alloy strip having a tensile strength after the annealing of 900 MPa or more, an elongation of 4% or more, and [longitudinal (coil-rolling direction)]/[transverse (direction perpendicular to the coil-rolling direction) elongation] of 0.4-1.0.